Advanced optical modulation formats are commonly referred to signals alternative to the common Non-Return-to-Zero (NRZ) and Return-to-Zero (RZ), offering improved performances and tolerances to optical linear and nonlinear physical effects. The binary NRZ format consists in transmitting a first of the two logical states (for example the 1) through a substantially constant optical signal over the whole bit time slot TB, and the second of the two logical states (for example the 0) through a reduced or absent optical power over the whole bit slot. In the common RZ modulation format, one of the two logical states corresponds to the presence of an optical pulse with proper optical power and duration shorter than the bit period TB, the second of the two logical states corresponds to the absence of pulses, or to a pulse with reduced power.
Key features for the success of advanced modulation technologies are the tolerances to linear and to nonlinear optical effects, the receiver sensitivity, the reachable signal extinction ratio, the number of modulators required at the transmitter, the electronics bandwidth as well as the maximum spectral efficiency in the wavelength division multiplexing (WDM) transmission. Local area optical networks (LAN) and metropolitan area networks (MAN) require modulation formats with large tolerances to chromatic dispersion, and low-cost transmitter/receiver devices.
Next generation WDM channels are planned to transmit at bit rates of R=40, 80 and 160 Gbit/s; the feasibility of commercial NRZ or RZ transmitters at those bit rates is not obvious, because the required bandwidth for the electronics and opto-electronics is comparable to R. The development of stable 40 GHz electronics has emerged in the last few years, and is still characterized by high production costs, while the development of electronics with cut-off frequency approaching to 80 or 160 GHz is still far to come.
In the last years, optical phase-shift keying (PSK) has been proposed for improving the tolerances to Kerr nonlinearity, optical signal-to-noise ratio (OSNR), WDM channel spacing and to chromatic dispersion. Optical phase is commonly modulated using a 2-level differential signal (DPSK), 4-level pre-encoded signal (DQPSK) or M-ary signal. Return-to-Zero Phase-Shift Keying (RZ-PSK) is characterized by the phase modulation of a train of optical pulses. In the common RZ-DPSK format, one of the two logical states corresponds to the presence of an optical pulse with arbitrary optical phase φ0 (rad), the second of the two logical states corresponds to a pulse with optical phase φ0+π. In the RZ-DQPSK format, two bits are contemporarily transmitted at each baud slot TB; one of the corresponding four logical states corresponds to the presence of an optical pulse with arbitrary optical phase φ0 (rad), the other logical states correspond to pulses with optical phase φ0+π/2, φ0+π and φ0+3π/2. The RZ power shaping advantageously eliminates the patterning effects due to the phase modulation, by reducing the optical power in correspondence of the phase transitions.
Differential quadrature phase-shift keying (DQPSK and RZ-DQPSK) has revealed one of the most promising modulation formats for metropolitan and long-haul applications; the transmission of a multi-level signal permits to reduce the transmitter/receiver electrical and optical bandwidth, as well as the optical pulse repetition rate, increasing the tolerances to chromatic dispersion. Paper [R. A. Griffin, A. C. Carter, “Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission”, OFC '02, vol. WX6, 2002] introduces a new DQPSK transmitter scheme based on a Mach-Zehnder super-structure, including 2 phase modulators; a further external intensity shaper modulator may be added to the proposed transmitter to generate a RZ-DQPSK optical signal. Patent application publication no. US 2005/0074245 A1, by R. Griffin, describes the above DQPSK transmitter with Mach-Zehnder super-structure. Paper [P. S. Cho, G. Harston, C. J. Kerr, A. S. Greenblatt, A. Kaplan, Y. Achiam, G. Levy-Yurista, M. Margalit, Y. Gross, J. B. Khurgin, “Investigation of 2-b/s/Hz 40-Gb/s DWDM transmission over 4×100 km SMF-28 fiber using RZ-DQPSK and polarization multiplexing”, IEEE Photonics Technology Letters, vol. 16, pp. 656-659, Feburary 2004] investigates experimentally the use of polarization-time interleaving with RZ-DQPSK signals; RZ-DQPSK was generated using an intensity shaper modulator, and two Lithium Niobate Mach-Zehnder modulators in push-pull configuration, biased at the minimum transmission point. Paper [K. P. Ho, H. W. Cuei, “Generation of arbitrary quadrature signals using one dual-drive modulator”, J. of Lightwave Technology, vol. 23, pp. 764-770, February 2005] proposes the generation of NRZ QAM and QPSK optical signals using a single dual-drive Mach-Zehnder modulator. RZ-DQPSK is produced by an intensity shaper modulator cascaded to a dual-drive modulator biased at the minimum transmission point. Paper [M. Ohm, T. Freckmann, “Comparison of different DQPSK transmitters with NRZ and RZ impulse shaping”, proc. of LEOS '04, vol. ThB2, 2004] investigates numerically three DQPSK transmitter schemes with NRZ optical power; one of the 3 transmitters employs a single phase modulator driven by a 4-level electrical signal. No RZ amplitude shaping is investigated. Patent no. U.S. Pat. No. 6,798,557 B1, by A. Leven, introduces a binary phase modulator with arbitrary phase shift, employing a Mach-Zehnder super-structure with a single phase modulator. The device is suitable to be cascaded with other Mach-Zehnder super-structures to generate a M-ary phase-modulated optical signal. RZ pulse shaping may be added using a further external shaper modulator. Patent no. U.S. Pat. No. 6,271,950 B1, by P. B. Hansen and T. N. Nielsen, describes an optical transmitter with N cascaded phase modulators, suitable for the time division multiplexing (TDM) of N DPSK tributaries.
The modulation formats and transmitter/receiver schemes proposed above require one or two optical phase modulators to generate a DQPSK optical signal, and one to n=log2M phase modulators for a M-ary signal. For all the transmitter schemes presented in the prior art, a further optical intensity modulator is required to shape the optical pulses in the form of a pulse train (RZ-DPSK, RZ-DQPSK etc.). The complexity of the proposed transmitters, and the high costs for each optical modulator reduce their applicability to metropolitan and local area networks, where cost-effective devices are a primary requisite.